Miniaturization of sensor circuits has become an important challenge. RFID tags, for instance, generally require coils external to the chip (although, some versions exist with coils integrated with the chip) for operation at short range. In one example, a small RFID chip measures 50 μm×50 μm and includes a post-processed coil, as described in “A 0.05×0.05 mm2 RFID Chip with Easily Scaled-Down ID-Memory,” by M. Usami et al., Solid-State Circuits IEEE International Conference—ISSCC, pp. 482-483, 2007.
Some circuits for photometric imaging applications, such as the circuits described in “Analysis of Dynamic Range, Linearity, and Noise of a Pulse-Frequency Modulation Pixel,” by Tsung-Hsun Tsai et al., IEEE Transactions On Electron Devices, Vol. 59, No. 10, pp. 2675-2681, July 2012, do not have a complete self-contained circuit inside the small area of the sensor. These circuits depend on a stable reference and power supply available from outside the sensor area.
Some biosensor modules use a fluorescent sensing mechanism and are powered by light, and may be as small as 250 μm×250 μm, as described in “Microtransponders, the miniature RFID electronic chips, as platforms for cell growth in cytotoxicity assays,” by W. Mandecki et al., 2006 International Society for Analytical Cytology, pp. 1097-1105. These biosensor modules, however, only transmit an identification code and do not contain any sensor readout mechanism on chip. Further, these modules rely on an external unit to read out the fluorescent sensor signal optically, which limits the transduction mechanism to photometric and limit the types of molecules that can be sensed.
There is a motivation to further miniaturize current sensor circuits in order to enable further applications.